They are silver in the book. Hollywood changed them to ruby red to take advantage of the new Technicolor used in the movie version, evidently ignorant of the meaning of the silver. Byran and many other Greenbackers (monetary reformers supporting use of debt-free US Notes like Lincoln’s Greenbacks to increase the money supply and thereby end the depression then) shifted their tactics to the promotion of adding silver to the lawful coinage of America (i.e., to promoting a bi-metallic standard rather than the theoretically purer, fiat Greenbacks) when they realized they could thereby gain the backing of the powerful silver mining interests and still increase the money supply (without debt) to more than just gold. Silver thus became a symbol of overcoming a purely gold standard with the limited money supply and banker control that resulted in. Hence the silver slippers were extremely important in the book, as silver coin was in reality.